Why does the primary structure information in the datasheet contain the description “including an N-terminal methionine” for some cytokines, but not for all?

The active natural form of cytokines does not contain an N-terminal methionine. However, when proteins are expressed in a recombinant form, an N-terminal methionine is often beneficial for the recombinant manufacturing of proteins, and many commercial cytokine preparations have an N-terminal methionine. Our goal is to manufacture authentic cytokines that are most similar to the natural form, and therefore most of our cytokines do not have an N-terminal methionine. Unfortunately, sometimes it is technically not possible, and these cases are marked by the description “including an N-terminal methionine”.